1. Field of the Invention
This invention relates to switching power supplies, and more particularly, to accounting for the effects of harmonic spurs produced by switching power supplies.
2. Description of the Related Art
Switching power supplies are well known in the electronic arts. A switching power supply may be used in place of a linear power supply in order to provide a relatively stable supply voltage. The supply voltage is typically generated from a greater input voltage. For example, a switching power supply can provide a supply voltage of 2.5 volts by switching alternately switching on and off a 5 volt input voltage at a 50% duty cycle. By maintaining a 50% duty cycle, an average voltage of 2.5 volts can be achieved. In another example, a supply voltage of 3 volts may be provided by switching the input voltage of 5 volts on and off with a 60% duty cycle (i.e. on 60% of the time, off the remaining 40%). If the switching frequency is high enough, once filtered, the voltage produced by a switching power supply appears as a steady voltage to the load at which it is received. Furthermore, whereas a typical linear voltage regulator may dissipate excess power in order to step down the voltage, a switching power supply typically dissipates relatively less power by alternating between an efficient on state, and an off state that dissipates very little power. Switching power supplies may therefore be useful in applications where power conservation is an important design parameter, such as portable devices that utilize a battery as a power source.
Although switching power supplies can be extremely useful in a wide variety of environments, they do have certain disadvantages. One such disadvantage is the presence of harmonic spurs. Such harmonic spurs occur at multiples of the switching frequency, i.e., the frequency at which the power supply is switched on and off. For example, a switching power supply operating at 5 MHz may produce harmonics at every 5 MHz multiple thereof (i.e. at 10 MHz, at 15 MHz, at 20 MHz, and so forth). These harmonic spurs can adversely affect operations of an electronic system if they fall within a frequency band of interest. Using the same switching frequency value of 5 MHz as an example, a radio receiver tuned to receive signals at 1 GHz may be adversely affected by the 200th harmonic spur from the 5 MHz switching frequency. If the harmonic spur is strong enough, it may interfere with the receiver's ability to detect and/or demodulate an in-band signal.
In order to overcome the effect of such harmonic spurs, various techniques may be employed. Many switching power supplies already use low pass filters to remove high-frequency components that result from the on-off transition of the power supply switch. Low pass filters may also be employed to remove signal energy at the frequency of the resultant harmonic spurs. Other techniques may also be used to reduce the signal energy of harmonic spurs, such as buffering and various types of electrical isolation.